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NTC Overview
NTC 0xA9 Current Offset Reasonableness

Description: The Current Offset Reasonableness Diagnostic detects errors in the current measurement circuit that may lead to current feedback offset values that may cause performance and safety requirement violations.

Diagnostic Overview:  If a large current offset error occurs in a phase current measurement, the closed loop current control system adjusts the motor voltage so that the measurement of motor current matches the motor current command. If a fault exists that causes the motor current measurement to be incorrect then the motor voltage will thus be incorrect. This failure mode results in motor position-dependent motor torque or current errors which may be perceived as a large torque ripple at the motor shaft, and potentially larger than rated motor currents.  When the torque ripple caused by the phase current measurement offset error becomes large, it can produce motor torque in the opposite direction from the motor torque command resulting in difficulty moving a motor from specific motor positions.

The Current Offset Reasonableness method generates an expected motor voltage command based on a reference current command and a set of estimated motor parameters of the motor. The actual motor voltage command is  based on the reference current command and a set of measured motor current signals.   An error signal is calculated based on the expected motor voltage command and the actual motor voltage command. The method transforms the error signal into a stator reference frame (necessary in order for the current measurement offset error to appear as an offset in the voltage error signal) and diagnoses whether the set of measured motor current signals is faulty based on the transformed error signal.

Typical Fault Response:  F3 (Operator Notification)

 
(Most to Least Probable):

• •Current Offset calibration incorrect. 

• •Incorrect motor parameters for actual motor or motor parameter SW estimates being clamped by min/max calibration variables, 

• •External circuit  … 

• • –Current Measurement Subsystem  

• •More